两种串列方柱气动性能的试验研究

针对两种布置形式（水平布置和对角布置）的串列双方柱,通过同步测压风洞试验,在雷诺数为Re = 8.0×104、间距比为P/B = 1.75 ~ 5.00（其中P为方柱中心间距、B为方柱边长）条件下,得到了两种布置形式串列双方柱的表面风压,重点研究了对角串列双方柱的气动力、风压分布、Strouhal数等气动性能随方柱间距的变化规律,并与水平串列双方柱进行比较。水平串列双方柱的气动力在P/B = 3.00 ~ 3.50时会发生跳跃现象,下游方柱的平均阻力由负值突变为正值,而对角下游方柱平均阻力系数则均为负值。结果表明：当P/B<3.00时,对角串列双方柱的平均和脉动气动力系数、最大平均负压强度和脉动风压系数均大于水平串列双方柱,而当P/B > 3.00时则情况相反;对角串列双方柱的Strouhal数明显小于相同间距下的水平串列双方柱,且在P/B <3.00时对角串列双方柱的升力功率谱出现了多个峰值。     

Unsteady computation of flow field and convective heat transfer over tandem cylinders at subcritical Reynolds numbers

Unsteady flow and convective heat transfer over single and two tandem cylinders at constant-heat-flux condition in subcritical range of Reynolds number was numerically investigated. Two-dimensional computations were performed by adopting 3-equation k-kl-ω turbulence model using a commercial software FLUENT®. The aim was to investigate the capabilities of k-kl-ω turbulence model for collective flow and heat transport conditions past cylindrical bodies and then to identify a critical spacing ratio for the maximum heat transport. The center-to-center spacing ratio (L/D) was varied in the range from 1.2 to 4.0. Instantaneous path lines and vorticity contours were generated to interpret the interaction of shear layer and vortices from upstream cylinder with the downstream cylinder. Comparison of pressure coefficients, fluctuating and average lift as well as drag coefficients, Strouhal number and the local and average Nusselt numbers with the available literatures indicated a reasonably good agreement. The combined outcome of flow field and heat transfer study revealed a critical spacing ratio of L/D = 2.2. Based on the present investigation, a correlation has been suggested to calculate overall average Nusselt number of the two cylinders placed in tandem.

     

串列三圆柱绕流的时均压力分布与气动力

运用刚性模型测压风洞试验方法对单圆柱、不同间距串列双圆柱和串列三圆柱绕流的时均压力分布与气动力进行了研究。首先,进行单圆柱模型和不同间距串列双圆柱模型的绕流试验,试验的雷诺数为3.4×104;其次,通过与单圆柱进行对比,讨论了气动干扰对串列三圆柱时均压力分布与时均阻力的影响规律;最后,通过与串列双圆柱进行对比,讨论了圆柱的数量对干扰规律的影响。试验结果发现,串列三圆柱的绕流存在两个完全不同的流态,其切换的临界间距(L/D)cr在3.5～4.0之间,两个流态下的时均压力分布与时均阻力存在明显的差异。本研究可对实际工程中串列圆柱结构的风荷载取值提供参考。     

Effect of Reynolds numbers on flow past four square cylinders in an in-line square configuration for different gap spacings

In this paper two-dimensional (2-D) numerical investigation of flow past four square cylinders in an in-line square configuration are performed using the lattice Boltzmann method. The gap spacing g = s/d is set at 1, 3 and 6 and Reynolds number ranging from Re = 60 to 175. We observed four distinct wake patterns: (i) a steady wake pattern (Re = 60 and g = 1); (ii) a stable shielding wake pattern (80 ≤ Re ≤ 175 and g = 1); (iii) a wiggling shielding wake pattern (60 ≤ Re ≤ 175 and g = 3) and (iv) a vortex shedding wake pattern (60 ≤ Re ≤ 175 and g = 6). At g = 1, the Reynolds number is observed to have a strong effect on the wake patterns. It is also found that at g = 1, the secondary cylinder interaction frequency significantly contributes for drag and lift coefficients signal. It is found that the primary vortex shedding frequency dominates the flow and the role of secondary cylinder interaction frequency almost vanish at g = 6. It is observed that the jet between the gaps strongly influenced the wake interaction for different gap spacing and Reynolds number combination. To fully understand the wake transformations the details vorticity contour visualization, power spectra of lift coefficient signal and time signal analysis of drag and lift coefficients also presented in this paper.     

Numerical simulation of flow past rectangular cylinders with different aspect ratios using the incompressible lattice Boltzmann method

This paper presents a numerical study of a uniform flow past a rectangular cylinder using the incompressible lattice Boltzmann method (ILBM). Firstly, we use the ILBM to simulate the flow past a square cylinder symmetrically placed in a two-dimensional channel and results are validated against the well-resolved results obtained using finite-difference method and finite-volume method. Secondly, the effects of the aspect ratio defined as R = width/height on the fluid forces, vortex shedding frequency and the flow structures in the wake are investigated. Aspect ratios ranging from 0.15 to 4.00 and four Reynolds numbers Re = 100, 150, 200 and 250 are selected for the investigation. The results show that the effects of aspect ratio on physical quantities such as drag and lift coefficients, Strouhal number and the vortex shedding mechanism are very notable in the range between 0 and 2. In general, the drag coefficient decreases with the aspect ratio and the decreasing rate is more distinct in the range of 0.15 ≤ R ≤ 2.0. There is no local maximum found at around R = 0.6 in the drag coefficient as reported for higher Reynolds numbers in the literature. However the root-mean-square value of the lift coefficient shows a maximum value at R ≈ 0.5 for all Reynolds numbers selected. The variation of Strouhal number with R appears to be different for four selected Reynolds numbers. Especially for Re = 250, a discontinuity in St, as has been observed for higher Reynolds numbers, is observed at around R = 1.45 where multiple peaks are found in the result of Fourier spectrum analysis of the lift force and irregular vortex shedding behavior with no fixed shedding frequency is observed from the instantaneous vorticity contours. Such discontinuity is not observed for Re = 100, 150 and 200. The present results using the LBM are compared with some existing experimental data and numerical studies. The comparison shows that the LBM can capture the characteristics of the bluff body flow well and is a useful tool for bluff body flow studies.     

Experimental study of effects of circular-cross-section riblets on the aerodynamic performance of Risø airfoil at transient flow regime

New drag reduction methods have received much attention due to the importance of drag reduction in airplanes and wind turbines. One of the ways for drag reduction is the use of riblets. We investigated the effects of riblets on the aerodynamic performance of the Risø airfoil quantitatively. By installing a load cell and using the one-sided force measurement method, the drag and lift coefficients of the Risø airfoil were measured in two modes: With and without riblets at three different arrangements. The shape of riblets is a circularcross- section and the ratio of riblets’ diameter to the airfoil chord is equal to 0.005. The tests were carried out in transient flow regime (Two Reynolds numbers of 2.02×10⁵ and 1.4×10⁵), and at attack angles from 0 to 20 degrees. The results indicate that the extent of the riblets effect on the aerodynamic performance of the airfoil depends on the angle of attack, Reynolds number, and arrangement of the riblets on the airfoil. The maximum drag reduction at the Reynolds numbers of 2.02×10⁵ and 1.4×10⁵ is about 29.7 % and 54 %, respectively, that occurs at an attack angle of 7 degrees for both two Reynolds numbers.     

Numerical study on laminar flow over three side-by-side cylinders

The present study has numerically investigated two-dimensional flow over three circular cylinders in an equidistant side-by-side arrangement at a low Reynolds number. For the study, numerical simulations are performed, using the immersed boundary method, in the range ofg* < 5 at Re=100, whereg* is the spacing between two adjacent cylinder surfaces divided by the cylinder diameter. Results show that the flow characteristics significantly depend on the gap spacing and a total of five kinds of wake patterns are observed over the range: modulationsynchronized (g*≥2), inphase-synchronized(g*≈l.5), flip-flopping (0.3<g*≲1.2), deflected (g*≈0.3), and single bluff-body patterns (g*<0.3). Moreover, the parallel and symmetric modes are also observed depending ong* in the regime of the flip-flopping pattern. The corresponding flow fields and statistics are presented to verify the observations.     

Flow characteristics around a row of circular and wavy cylinders

The near-wake and flow interference around a row of five circular cylinders, staggered wavy cylinders and nonstaggered wavy cylinders are investigated experimentally by using PIV and LIF techniques. The effects and characteristics of employing wavy cylinders instead of circular cylinders are discussed. The cylinders were arranged atT/d=1.5 withRe ranging from 125 to 40000. Results showed the staggered wavy cylinders give rise to a more stable flow pattern with less fluctuation and longer wake vortex closure length.     

A study of flow patterns for staggered cylinders at low Reynolds number by spectral element method

This study investigates the pattern of flow past two staggered array cylinders using the spectral element method by varying the distance between the cylinders and the angle of incidence (α) at low Reynolds numbers (Re = 100-800). Six flow patterns are identified as Shear layer reattachment (SLR), Induced separation (IS), Vortex impingement (VI), Synchronized vortex shedding (SVS), Vortex pairing and enveloping (VPE), and Vortex pairing splitting and enveloping (VPSE). These flow patterns can be transformed from one to another by changing the distance between the cylinders, the angle of incidence, or Re. SLR, IS and VI flow patterns appear in regimes with small angles of incidence (i.e., α ≤ 30° ) and hold only a single von Karman vortex shedding in a wake with one shedding frequency. SVS, VPE and VPSE flow patterns appear in regimes with large angles of incidence (i.e., 30° ≤ α ≤ 50° ) and present two synchronized von Karman vortices. Quantitative analyses and physical interpretation are also conducted to determine the generation mechanisms of the said flow patterns.     

Unsteady Lift and Drag Forces Acting on the Elliptic Cylinder

A parametric study has been accomplished to figure out the effects of elliptic cylinder thickness, angle of attack, and Reynolds number on the unsteady lift and drag forces exerted on the elliptic cylinder. A two-dimensional incompressible Navier-Stokes flow solver is developed based on the SIMPLER method in the body-intrinsic coordinates system to analyze the unsteady viscous flow over elliptic cylinder. Thickness-to-chord ratios of 0.2, 0.4, and 0.6 elliptic cylinders are simulated at different Reynolds numbers of 400 and 600, and angles of attack of 10°, 20°, and 30°. Through this study, it is observed that the elliptic cylinder thickness, angle of attack, and Reynolds number are very important parameters to decide the lift and drag forces. All these parameters also affect significantly the frequencies of the unsteady force oscillations.     

Unsteady viscous flow over elliptic cylinders at various thickness with different reynolds numbers

Two-dimensional incompressible Navier-Stokes equations are solved using SIMPLER method in the intrinsic curvilinear coordinates system to study the unsteady viscous flow physics over two-dimensional ellipses Unsteady viscous flows over various thickness-to-chord ratios of 06, 08, 10, and 1 2 elliptic cylinders are simulated at different Reynolds numbers of 200, 400, and 1,000 This study is focused on the understanding the effects of Reynolds number and elliptic cylinder thickness on the drag and lift forces The present numerical solutions are compared with available experimental and numerical results and show a good agicement Through this study, it is observed that the Reynolds number and the cylinder thickness affect significantly the frequencies of the force oscillations as well as the mean values and the amplitudes of the drag and lift forces     

Numerical solution of flow past two circular cylinders in different arrangement rotating in various directions

The combined effects of different rotation types and the Reynolds number on the flow past two rotating circular cylinders about their axes in different arrangement (Side-by-side and tandem) were considered at a range of 520 ≤ Re ≤ 1570 and 0 ≤ ω ≤ 4 (ω is the rotational speed) at one gap spacing of L/D = 2 for a side-by-side arrangement, ω = 0, 2000 ≤ Re ≤ 21000 and L/D = 2 and 4/3 for tandem arrangement (L and D are the distance between the centers of two cylinders and the cylinder diameter, respectively). The results show that the variation of both rotation speed and Reynolds number have an important role in changing the pattern of vortex shedding. As the rotational speed further increases, the separation phenomenon in the boundary layers disappears at the attachment rotational speed. Regardless of Reynolds number, as ω increases, the lift decreases for up and down cylinders while the drag decreases for up cylinder and increases for down cylinder. Quantitative information is highlighted about the flow variables such as the pressure coefficient the Stanton number, the skin friction factor and wall viscous coefficient of the cylinders.     

Numerical simulation of the flow around two circular airfoils positioned across the stream using the LS-STAG method

The flow around two circular airfoils positioned side by side (across the stream) at different distances between the airfoils is considered. The LS-STAG method, which belongs to the class of immersed boundary method is used for numerical simulation that makes it possible to calculate using rather coarse meshes. Stationary and trigger flow regimes have been simulated, as well as the patterns of flow with synchronization of two vortex trails in-phase and in anti-phase. For each regime, the typical dependences of coefficients of drag force and lift force on the time are presented. The obtained results are in good agreement with the known experimental and computational data.     

Numerical investigation of fluid flow past a square cylinder using upstream,downstream and dual splitter plates

A two-dimensional numerical study is carried out to analyze the drag reduction and vortex shedding suppression behind a square cylinder in presence of splitter plate arranged in upstream, downstream and both upstream and downstream location at low Reynolds number (Re = 160). Computations are performed using a Single relaxation time lattice Boltzmann method (SRT-LBM). Firstly, the code is validated for flow past a single square cylinder. The obtained results are compared to those available in literature and found to be in good agreement. Numerical simulations are performed in the ranges of 1 ≤ L ≤ 4 and 0 ≤ g ≤ 7, where L and g are the length of splitter plate and gap spacing between the splitter plate and main square cylinder, respectively. The effect of these parameters on the vortex shedding frequency, time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualization and force exerted on the cylinder are quantified together with the observed flow patterns around the main cylinder and within the gap spacings. The observed results are also compared with a single square cylinder without splitter plate. We found that at some combinations of L and g, the mean drag coefficient and Strouhal number reach either its maximum or minimum value. It is found that the drag is reduced up to 62.2 %, 13.3 % and 70.2 % for upstream, downstream and dual splitter plates, respectively as compared to a single square cylinder (without splitter plate). In addition, in this paper we also discussed the applications of SRT-LBM for suppression of vortex shedding and reduction of the drag coefficients.     

Drag force control of flow over wavy cylinders at low reynolds number

Three-dimensional numerical simulations on the laminar flow around a circular cylinder with different diameter along the spanwise leading to a type of sinusoidal waviness, named wavy cylinder are performed at low Reynolds number. A series of wavy cylinders with different combinations of spanwise wavelength and wave amplitude are conducted at a Reynolds number of 100. The optimal range of wavelength and the effect of wave amplitude are obtained. The results show that the 3-D free shear layers from the cylinder are more difficult to roll up to vortex and hence the wake formation lengths of some typical wavy cylinders are larger than that of the circular cylinder and in some cases the free shear layers even do not roll up into vortex behind the cylinder. The mean drag coefficients of the typical wavy cylinders are less than that of a corresponding circular cylinder with the same mean diameter; also the fluctuating lift coefficients are reduced. The reduction of mean drag coefficient and fluctuating lift coefficient of wavy cylinder increases with the value of wavy amplitude. Furthermore, a typical wavy cylinder model at Re=150 is also simulated and found that the control of flow induced vibration by modifing the spanwise wavelength of cylinder has a relationship with the variation of Reynolds number.     

低含气率气液两相流涡街特性研究

气液两相涡街现象广泛存在于生产实际中,由于相间作用,研究变得较为复杂。通过数值模拟和实流试验,对水平管低含气率情况下气相对两相涡街的影响进行研究。建立三维计算流体力学（Computational fluid dynamics, CFD）仿真模型,采用VOF多相流模型和RNG k-e 湍流模型,模拟气液两相钝体绕流。仿真与试验结果在定量频率上具有良好的一致性。结合仿真与试验结果,从近尾迹流场与涡拓扑、斯特劳哈尔数两方面,对不同含气率两相涡街特性进行对比研究。结果表明,在稳定涡街范围内,随着气相含率增大,对涡街起重要作用的滞止区长度增大,导致斯特劳哈尔数线性减小;同时,涡街周期性和信号质量变差,涡街能量降低,这是由于涡中心吸入密度小、速度大的气泡,造成涡的旋转能量降低,进而影响涡街的稳定性。     

Design and fabrication of fluid flow characteristic controllable trawl door using a trailing edge flap

A novel trawl door with a trailing edge flap and a trawl door with a hydrophobic surface are proposed to improve the hydrodynamic characteristics of trawl doors. In the former, the flap angle was designed to accommodate rapid changes in water flow velocity, increase the lift force under low-speed conditions, and decrease drag force under high-speed conditions. An evaluation of the hydrodynamic characteristics at different flap angles was conducted through numerical analyses and experimentation. The design of the trawl door with coated hydrophobic surface aims to reduce flow resistance. Both trawl door designs were validated in a circulating water channel ex-periment. At these angles of the trailing edge flap, a 31 % decrement in drag force was observed in the circulating water channel experi-ment, and a 21.3 % decrement in flow resistance was observed when the surface of the trawl door was coated with hydrophobic surface materials.

     

Slip flow over micron-sized spherical particles at intermediate Reynolds numbers

A numerical investigation has been performed to identify the rarefaction effects on the flow structure of an isolated micron-sized spherical particle. An isothermal sphere in the slip flow regime 10^?3 ?? Kn ?? 10^?1 at intermediate Reynolds numbers (1 ?? Re ?? 50) is considered. The Navier-Stokes equations are solved by a control volume technique in conjunction with the velocity slip boundary condition. It was found that the wake region can shrink considerably as the Knudsen number increases. Furthermore, the skin friction and pressure drag coefficients decrease as the Knudsen number increases due to the reduction in normal velocity gradients and shrinkage of the wake region, respectively. Engineering correlations for predicting the total drag coefficient in the slip flow regime are presented.     

Large eddy simulation of flow around two side-by-side spheres

Large eddy simulation, using a dynamic Smagorinsky sub-grid scale, is used for the prediction of flow structures around two identical spheres fixed side-by-side at a subcritical flow regime with the Reynolds number equal to 5,000. The QUICK discretization method is applied to discretize the convection terms of the Navier-Stokes equation by means of the finite volume approach. This work focuses mainly on the wake structures downstream of the two interactive spheres located at three various dimensionless separation distances between spheres such as G/D = 1.50, 2.00, and 3.00. The obtained results revealed that the interaction between wakes affects the flow structures downstream of spheres. The rate of this interaction is strongly altered as a function of separation distance. On the other hand, some flow data such as mean drag and mean lift coefficients are affected due to the wake interactions. Finally, examination of the Reynolds stress variation along the different lateral axis, L/D, revealed that the nozzle effect does not play a significant role on the turbulence characteristics beyond the G/D = 3.00.     

A study on the flow characteristics between two circular cylinders in cross flow by ldv and holographic interferometry

Experimental investigation of the fluid flow and thermal flow pattern around two circular cylinders in cross flow has been carried out. Reynolds number was varied in the range of 100≦Re≦1000 and the distance between the two cylinders in the interval of 2≦L/D≦4. Velocity and turbulence intensity distributions in the isothermal distribution in the whole x−y thermal flow field was obtained by double-exposure holographic interferometry. The influences of Reynolds number and the gap spacing between the two cylinders were investigated. The characteristic flow pattern was found to depend on the distance between the two cylinders. WhenL≦3D, the wake region between the two cylinders became quasi-stationary.